The present invention relates to a hydraulic control system for a continuously variable transmission for a motor vehicle, and more particularly to a control system for controlling flow rate of oil in the transmission.
A known control system for a continuously variable transmission comprises an endless belt running over a primary pulley and a secondary pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary a running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a secondary pressure control valve and a primary pressure control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The system is employed with an electronic control system for controlling valves of the hydraulic circuit.
Flow rate of the oil from the oil pump is regulated by the secondary pressure control valve to produce a secondary pressure. The secondary pressure is reduced by the primary valve to produce a primary pressure. The primary pressure is applied to a primary cylinder of the primary pulley, and the secondary pressure is applied to a secondary cylinder of the secondary pulley for controlling transmission ratio. The flow rate of the oil is regulated in accordance with the various conditions. If the flow rate and the primary pressure largely change by a transmission control signal, the changes of the flow rate and the pressure effect the secondary pressure. Namely, when the primary pressure is increased at upshift of the transmission to expand the primary cylinder, there occur increases of compression pressure of air in the cylinder, leakage of the oil, and deformation of the cylinder. Therefore, it is necessary to supply the oil at a larger flow rate than that of a necessary pressure in the primary cylinder, as shown in FIG. 3. In particular, this phenomenon of requiring much oil is remarkable when the primary pressure rises from zero.
If a large amount of the primary pressure is required, the oil flow rate applied to the primary cylinder becomes unbalance. As a result, the secondary pressure reduces to cause belt slipping and malfunction of the transmission. To eliminate these disadvantages, it is necessary to control the transmission with properly maintaining the balance of the oil flow rate between the primary system and the secondary system.
Japanese Patent Application Laid-open 61-74951 discloses a hydraulic system for a continuously variable belt-drive transmission. The system has a regulator valve provided between an oil pump and a primary cylinder, and a bypass provided around the regulator valve. When oil leaks from the cylinder, a corresponding amount of the oil is supplied through the bypass to the cylinder.
However, since a large amount of the oil is supplied only to the primary cylinder, the secondary pressure is liable to reduce. Therefore, if the transmission is upshifted at a low engine speed, where flow rate of the oil is small, the secondary pressure is largely reduced.